The present invention can be applied to a broad range of semiconductor devices, although it is especially effective in light-receiving elements such as photodiodes and solar cells. The background of the invention is described below with reference to solar cells as a specific example of the prior art.
A conventional solar cell structure with a p-type base has a negative electrode that is typically on the front-side or sun side of the cell and a positive electrode on the back side. Radiation of an appropriate wavelength falling on a p-n junction of a semiconductor body serves as a source of external energy to generate hole-electron pairs in that body. Because of the potential difference which exists at a p-n junction, holes and electrons move across the junction in opposite directions and thereby give rise to flow of an electric current that is capable of delivering power to an external circuit. Most solar cells are in the form of a silicon wafer that has been metallized, i.e., provided with metal electrodes that are electrically conductive. Typically thick film pastes are screen printed onto substrate and fired to form the electrodes.
An example of this method of production is described below in conjunction with FIGS. 1A-1F.
FIG. 1A shows a single crystal or multi-crystalline p-type silicon substrate 10.
In FIG. 1B, an n-type diffusion layer 20 of the reverse conductivity type is formed by the thermal diffusion of phosphorus using phosphorus oxychloride as the phosphorus source. In the absence of any particular modifications, the diffusion layer 20 is formed over the entire surface of the silicon p-type substrate 10. The depth of the diffusion layer can be varied by controlling the diffusion temperature and time, and is generally formed in a thickness range of about 0.3 to 0.5 microns. The n-type diffusion layer may have a sheet resistivity of several tens of ohms per square up to about 120 ohms per square.
After protecting the front surface of this diffusion layer with a resist or the like, as shown in FIG. 1C the diffusion layer 20 is removed from the rest of the surfaces by etching so that it remains only on the front surface. The resist is then removed using an organic solvent or the like.
Then, as shown in FIG. 1D an insulating layer 30 which also functions as an anti-reflection coating is formed on the n-type diffusion layer 20. The insulating layer is commonly silicon nitride, but can also be a SiNx:H film (i.e., the insulating film comprises hydrogen for passivation during subsequent firing processing), a titanium oxide film, a silicon oxide film, or a silicon oxide/titanium oxide film. A thickness of about 700 to 900 Å of a silicon nitride film is suitable for a refractive index of about 1.9 to 2.0. Deposition of the insulating layer 30 can be by sputtering, chemical vapor deposition, or other methods.
Next, electrodes are formed. As shown in FIG. 1E, a silver paste 500 for the front electrode is screen printed on the silicon nitride film 30 and then dried. In addition, a back side silver or silver/aluminum paste 70, and an aluminum paste 60 are then screen printed onto the back side of the substrate and successively dried. Firing is carried out in an infrared furnace at a temperature range of approximately 750 to 850° C. for a period of from several seconds to several tens of minutes.
Consequently, as shown in FIG. 1F, during firing, aluminum diffuses from the aluminum paste 60 into the silicon substrate 10 on the back side thereby forming a p+ layer 40 containing a high concentration of aluminum dopant. This layer is generally called the back surface field (BSF) layer, and helps to improve the energy conversion efficiency of the solar cell.
Firing converts the dried aluminum paste 60 to an aluminum back electrode 61. The back side silver or silver/aluminum paste 70 is fired at the same time, becoming a silver or silver/aluminum back electrode, 71. During firing, the boundary between the back side aluminum and the back side silver or silver/aluminum assumes the state of an alloy, thereby achieving electrical connection. Most areas of the back electrode are occupied by the aluminum electrode 61, owing in part to the need to form a p+ layer 40. Because soldering to an aluminum electrode is impossible, the silver or silver/aluminum back electrode 71 is formed over portions of the back side as an electrode for interconnecting solar cells by means of copper ribbon or the like. In addition, the front side silver paste 500 sinters and penetrates through the silicon nitride film 30 during firin, and thereby achieves electrical contact with the n-type layer 20. This type of process is generally called “fire through.” The fired electrode 501 of FIG. 1F clearly shows the result of the fire through.
There is an on-going effort to provide thick film paste compositions that have reduced amounts of silver and are Pb-free while at the same time maintaining electrical performance and other relevant properties of the resulting electrodes and devices. The present invention provides a Ag paste composition that simultaneously provides a Pb-free system with lower amounts of Ag while still maintaining electrical and mechanical performance.